When producing semiconductor wafers, it is necessary to grind a predetermined profile or bevel on the perimeter of the wafer. Typically, this step is carried out on a machine known as an edge grinder, which includes a rotatable chuck mounted on top of a non-rotatable assembly known as a theta unit. The non-rotatable assembly is called a theta unit because the chuck on the assembly rotates a wafer through an angle, commonly referred to as "theta." The semiconductor wafer is placed on the chuck and, as the chuck is rotated or otherwise displaced, the edge of the wafer is carried past a spinning grinding head to create the desired profile. Known edge grinders are disclosed in U.S. Pat. Nos. 5,185,965 and 4,864,779, both to Ozaki, and the disclosures of both are incorporated herein by reference in their entireties.
Because the perimeter of the wafer is rotated past the grinding head by the chuck, it is critical that the wafer be perfectly centered over the rotation axis of the chuck. If the wafer is off-center, a "grind-out" may occur, where the grinding, head does not contact the entire perimeter of the wafer because of the eccentric position of the wafer. Any grind-out on a wafer snakes the wafer unusable and thus reduces overall production. Once the wafers are off-center, the chuck must be re-aligned--a time-consuming task that causes manufacturing down-time. Semiconductor manufacturers continually search for ways to reduce the down-time due to re-alignment requirements.
During the grinding operation it is important to ensure that the wafer is kept at an even temperature to prevent warping. This is typically accomplished lby spraying the wafer with a coolant fluid, such as water. One problem with spraying the wafer with coolant is that the coolant finds its way underneath the chuck and into the theta unit. Contaminants and small wafer grindings that are carried by the coolant tend to degrade the seals within the theta unit, especially any seal designed to prevent fluid flow between the rotatable and non-rotatable portions of the edge grinder. When such a seal is degraded, the coolant may find its way into internal bearings of the theta unit, wash out the grease in which the bearings are packed, thereby destroying the internal bearings and causing the chuck to rotate off-center. Furthermore, fluid breaching the seals may also find its way into a motor positioned below the theta unit and damage a sensitive optical encoder that is essential to precise electronic control of the motor during grinding operations. At a minimum, fluid intrusion through the seals will result in a compromise in the precision of the theta unit. Additionally, intrusion of fluid across the theta unit will result in damage to the motor assembly.
Efforts to reduce fluid intrusion underneath the chuck have included inserting a drain collar or other draining device within the grinding machine. While such measures may reduce the amount of fluid intrusion, even a small amount of contaminant-laden fluid sitting on the seals degrade,; the seals over time.